dotnet/reactive · error · ArgumentNullException

Thrown when source is null (ArgumentNullException, param…

Error message

Thrown when source is null (ArgumentNullException, param name: source)

What it means

IgnoreElements enumerates a sequence and discards all of its elements, yielding nothing. Because the argument is validated eagerly in the public operator, passing a null source throws ArgumentNullException with paramName 'source' immediately rather than on MoveNext.

Solutions

Verify the source expression is non-null before invoking IgnoreElements; fix the producer so it returns an empty sequence instead of null.
Coalesce with ?? Enumerable.Empty<TSource>() at the call site.
If null signals a real problem, check and throw/log before calling, so the failure is attributable to the actual cause.

Example fix

// before
items.IgnoreElements().Run(); // items is null when the cache missed
// after
(items ?? Enumerable.Empty<Item>()).IgnoreElements().Run();

Defensive patterns

Strategy: validation

Validate before calling

if (source is null) source = Enumerable.Empty<TSource>();
source.IgnoreElements();

Type guard

static bool HasSource<T>(IEnumerable<T>? s) => s is not null;

Try / catch

try
{
    source.IgnoreElements();
}
catch (ArgumentNullException ex) when (ex.ParamName == "source")
{
    // source was null; log and continue with nothing to drain
}

Prevention

Coalesce nullable sources with ?? Enumerable.Empty<T>() before chaining Ix operators
Fix producers to return empty sequences, never null, for 'no data'
Enable nullable reference types to catch unassigned source variables

When it happens

Trigger: Calling source.IgnoreElements() (extension method on IEnumerable<TSource>) where source is null, e.g. someQuery.IgnoreElements() when someQuery came from a null-returning factory or deserialization.

Common situations: Chaining Ix operators onto results of methods that may return null (configuration lookups, optional service calls); using IgnoreElements purely for side effects (e.g. draining an observable-like source) where the source variable was never assigned.

Related errors

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/90e74a9a2b189d0d. Report an issue: GitHub.

Appendix: source

Thrown at Ix.NET/Source/System.Interactive/System/Linq/Operators/IgnoreElements.cs:20

// The .NET Foundation licenses this file to you under the MIT License.
// See the LICENSE file in the project root for more information. 

using System.Collections.Generic;

namespace System.Linq
{
    public static partial class EnumerableEx
    {
        /// <summary>
        /// Ignores all elements in the source sequence.
        /// </summary>
        /// <typeparam name="TSource">Source sequence element type.</typeparam>
        /// <param name="source">Source sequence.</param>
        /// <returns>Source sequence without its elements.</returns>
        public static IEnumerable<TSource> IgnoreElements<TSource>(this IEnumerable<TSource> source)
        {
            if (source == null)
                throw new ArgumentNullException(nameof(source));

            return IgnoreElementsCore(source);
        }

        private static IEnumerable<TSource> IgnoreElementsCore<TSource>(IEnumerable<TSource> source)
        {
            foreach (var _ in source)
                ;

            yield break;
        }
    }
}

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